Heat-flowable fluororesins having properties such as chemical resistance, nonstickiness, heat resistance, a low friction of coefficient and electrical insulation properties, are also capable of forming pinhole-free films and are thus useful as coating materials. Heat-flowable fluororesins used in such coating applications include, for example, copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl ether) (hereinafter referred to as PFA), copolymers of tetrafluoroethylene and hexafluoropropylene (FEP), and copolymers of tetrafluoroethylene and ethylene (ETFE). These resins are insoluble in water and organic solvents, and so cannot be used as solution-type coatings. Thus, these resins are applied onto substrates by various other means, including electrostatic application of powder coating compositions or used as liquid dispersions. Liquid dispersions can be water-based and stabilized with a surfactant and or can be based on organic liquids. Liquid dispersions are applied to substrates by such means as spray-coating, dipping, or curtain-coating followed by heating and fusing to generate film coatings.
The film coatings obtained from electrostatic application of powder coating compositions generally have a thickness of about 50 to 100 .mu.m, whereas dispersion coating compositions generally produce coated films having a thickness of 20 .mu.m. However, this level of thickness is not sufficient for applications which require corrosion prevention, which leads to the demand for developing coating compositions capable of forming thicker film coatings.
Japanese Patent Application 57-15607 discloses a fluororesin liquid dispersion for thick-film coating application comprising a heat-flowable fluororesin powder having an average particle size of 2 to 300 .mu.m and a void fraction of not more than 0.74 in a liquid dispersing medium having a surface tension of not more than 45 dyne/cm. This dispersion makes it possible to form thick, 500-.mu.m coats when the surface being coated is horizontal or level, but when the surface being coated is vertical or inclined at an angle, the fluororesin powder falls off the coated surface prior to baking and film formation.
In order to form thick coats on vertical surfaces as well, U.S. Pat. No. 5,502,097 discloses a fluororesin powder liquid dispersion for thick-film coating application comprising a heat-flowable fluororesin powder having an average particle size of 5 to 300 .mu.m and a porosity of not more than 0.74 and a total surface area of not more than 10 m.sup.2 /cm.sup.3 in a liquid dispersion medium having a surface tension of not more than 45 dyne/cm, and also containing an organic liquid having a boiling point of 150 to 340.degree. C. This invention enables a thick coat to be formed even when the surface being coated is vertical. However, because the flammability of the organic liquid in the dispersion presents a danger of explosion, this has imposed limits on the working environment.
Thus, there still remains a need, especially in applications requiring corrosion resistance, such as at chemical plants where the performance of a coating is approximately dependent on the thickness of a film, for coating compositions capable of forming thick coated films on vertical surfaces and which can be applied safely.